Demographic trends underscore the public health significance of research to decrease morbidity and mortality in adults 65 years of age and older. Societal and economic costs of diseases of old age will increase exponentially as the population ages. Because the immune response is a major factor in determining longevity, the development of novel, low-cost interventions to slow the rate of immunosenescence could lead to reduced morbidity, as well as substantial cost-savings. In the proposed randomized clinical trial (RCT), 200 older adults (65 years of age or older) will be randomly assigned to either an experimental intervention, Mindfulness Based Stress Reduction (MBSR), or a Living Well control condition. Effects on immunological, psychological, and physical outcomes will be examined. A primary outcome measure for this study is immune function. We will capitalize on the antibody response to a novel, benign antigen to which subjects will be immunologically naive. The use of a novel antigen will circumvent the real possibility that higher baseline antibody liters to, e.g., a booster vaccination will be associated with less new antibody production. This strategy will also allow us to define differences between treatment groups in the magnitude of the antibody response across multiple antigen doses, from suboptimal to optimal. The proposed study will examine two additional critical questions. The first concerns treatment moderators, and asks who among the elderly will benefit from MBSR. The second addresses behavioral, psychological, and physiological mediators of change, including behavioral adherence to meditative practice, increases in mindfulness, decreases in stress, and changes in brain electrical activity, as measured by electroencephalography (EEG). The Specific Aims are to examine: 1. The effects of Mindfulness Based Stress Reduction (MBSR) on immunological outcomes, perceived health, and psychological well-being in a sample of seniors 65 years of age and older. 2. Whether treatment effects are moderated by age, personality traits, physical health status, or depression. 3. The effects of behavioral, psychological, and physiological mediators of immune outcome. The findings of this RCT will have implications for the conceptualization, refinement, and dissemination of psychosocial interventions aimed at slowing the rate of immunosenescence.